Constant velocity joints are classified roughly into two types; the fixed type which allows only angular displacement between input and output shafts, and the slide type which allows angular displacement and axial displacement, each type being selected according to application and use conditions. Widely used as fixed type constant velocity joints are the Rzeppa type (hereinafter referred to as “BJ”) and undercut free type (hereinafter referred to as “UJ”). The BJ and UJ each comprises an outer ring having a plurality of curved ball grooves in the inner periphery, an inner ring having a plurality of curved ball grooves in the outer periphery, balls incorporated between the ball grooves of the outer ring and the ball grooves of the inner ring, and a cage for holding the balls. The ball groove center of the outer ring is positioned on the outer ring opening side with respect to the outer ring inner spherical surface center and the ball groove center of the inner ring is positioned on the outer ring innermost side with respect to the inner ring outer spherical surface center, the ball groove centers being offset axially by an equal distance in opposite directions. Therefore, the ball tracks defined by ball grooves of the outer and inner rings are in the form of a wedge gradually contracting or expanding from one to the other of the axial direction of the joint. In the BJ the entire region of each ball groove is curved, while in the UJ, one end of each ball groove straight is parallel with the axis.
Generally, a steering joint for automobiles uses two or more cardan joints. Since this joint provides ununiform velocity if used by itself, two or more joints are disposed to ensure that variation components cancel each other so as to secure uniform velocity property. For this reason, a problem exists in that the degree of freedom of design of vehicles is decreased. The use of constant velocity joints, which can secure uniform velocity property at optional angles, as steering shaft joints makes it possible to increase the degree of freedom of the design of vehicles; however, since constant velocity joints have a large play in the direction of rotation, it is feared that such play may cause degradation of steering feel or cause abnormal sounds, in the vicinity of vehicle straight travel. To solve this, Japanese Unexamined Patent Publication No. 2003-130082 proposes that a pre loading means be provided in the constant velocity joint to reduce track clearances. The track clearances mentioned herein means clearances between the ball tracks and the torque transmitting balls or more specifically clearances between the ball grooves of the outer ring and the torque transmitting balls and clearances between the inner ring ball grooves and the torque transmitting balls.
In fixed type constant velocity joints, there exist track clearances from the functional and processing aspect, and there also exist clearances between the inner spherical surface of the outer ring and the outer spherical surface of the cage and between the outer spherical surface of the inner ring and the inner spherical surface of the cage. The presence of these clearances makes it possible to fix one of the inner and outer rings and move the other radially or axially in the neutral state of the joint. The amount of movement at this time is called a radial clearance or an axial clearance depending on the direction of movement. These clearances largely influence the circumferential play (rotational backlash) between the inner and outer rings; specifically, the larger the track clearances, the larger the rotational backlash. For this reason, a certain degree of rotational backlash is unavoidable, so that this kind of fixed type constant velocity joint has not yet been generally employed in situations where rotational backlash should be avoided as in the steering devices of automobiles, for example.
The invention disclosed in Japanese Unexamined Patent Publication No. 2003-130082 is a fixed type constant velocity joint intended to reduce the rotational backlash; however, it has the possibility that hysteresis may increase depending on the conditions for attachment to vehicles and detract the steering stability in the vicinity of straight travel of vehicles.